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An Introduction to Bent's Rule

Last updated date: 18th Mar 2023
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Pauling, Coulson, Slater, and many others started applying the theories to molecule structure in the early 1930s. Pauling specifically established the idea of hybridisation, in which atomic s and p orbitals are joined to produce hybrid sp, sp2, and sp3 orbitals. The molecular geometry of essential molecules like methane was successfully explained using hybrid orbitals.

But in the 1940s, small differences from this idealised geometry were visible. It was suggested that hybridisation could produce orbitals with uneven s and p characters in order to understand such differences. The electronegativity of groups bound to carbon and the hybridisation of that carbon was linked, as stated by A. D. Walsh in 1947.

At last, Henry Bent produced a significant assessment of the literature in 1961 that connected molecule structure, central atom hybridisation, and electronegativities of the substituent. Bent's rule is named after this work. Bent's rule offers a qualitative approximation of how various hybridised orbitals must be built. Hence, this article provides in-depth information on Bent's rule and Bent's rule application, and certain examples.

What is Bent's Rule?

The link between the electronegative nature of substituents and the orbital hybridisations of the key atoms in molecules is described and explained by Bent's Rule.

Bent's rule states that the main atom attached to several groups in a molecule will hybridise such that the atomic s character concentrates in orbitals oriented toward electropositive groups, while the atomic p character concentrates in orbitals oriented toward electronegative groups.

An example of Bent’s rule is given below.

By altering the substituent groups, molecule geometry can be anticipated and described. The bond angle of Cl-X-Cl is lower than C-X-C in the molecule Me2XCl2 (X= can be groups like Sn, C, Ge, Pb, and Si). Additional p characteristics are concentrated upon the fundamental atom in X-Cl bonds than X-C ones due to the highly electronegative halogen group (Cl).

Bent's Rule Applications

Numerous facets of bonding, structure, spectra and chemical and physical properties of the diverse spectrum of molecules can be explained by Bent's rule. The following is a collection of a few significant applications of Bent's rule, which is well-known in Science.

  • Bond Angle

  • Bond Length

  • JCH Coupling Constants

  • Inductive Effects

Bond Angle

Bent's rule describes an alternate justification for why certain bond angles deviate from the optimal geometry. Initially, employing the molecules methane, ethylene, and acetylene, to Bent’s rule, it is possible to identify a correlation involving central atom hybridisation and bond angle.

The carbon atoms are sequentially pointing their sp3, sp2, and sp orbitals in the direction of the hydrogen groups. These are three substituents with bond angles of 109.5°, 120°, and 180°. This basic system shows that atomic orbitals hybridised with a greater p character possess a lower angle connecting them.

Bond Length

An atom's hybridisation can be correlated with the bond lengths it makes, just like bond angles. The length of the bond shortens as the s character of bonding orbitals increases. Bond lengths can be changed by including electronegative groups and altering the degree of hybridisation of the core atoms.

JCH Coupling Constants

The JCH coupling constants derived from NMR spectra may be the most precise method of measuring the s characteristic in a bonding orbital involving carbon and hydrogen. According to the concept, bonds containing stronger s characteristics will have substantially greater JCH values. The level of p characteristic focused toward the group grows along with the substituent's increased electronegativity.

Inductive Effects

Bent's rule can also be used to understand the inductive effect. Using variations in hybridisation, Bent's rule offers a mechanism for these outcomes because the inductive effect is the passage of charge via covalent bonding. Higher p character is redirected to the groups as their electronegativity rises, leaving higher s character within the bond involving the core carbon as well as the R group.

Since the s orbitals contain a higher electron density near the nuclei versus p orbitals, the C-R bond electron density will move to the carbon while the s character rises. This will increase the core carbon's ability to pull electrons from the R group. Therefore, as predicted by the inductive effect, the substituents' capacity to remove electrons has been shifted to the nearby carbon.

Interesting Facts

  • Improved descriptions of characteristics like molecule geometry and bond strength can be obtained by eliminating the presumption that all hybridised orbitals are comparable spn orbitals.

  • Bent’s rule has been suggested as a substitute for VSEPR theory.

  • Bent’s rule has the attributes of being more readily compatible with contemporary theories of bonding and possessing greater empirical backing.

  • Recently, the applicability of Bent's rule considering 75 different bond kinds connecting the core group elements was investigated. Patterns in orbital hybridisation are substantially influenced by orbital size and electronegativity for bonds containing the bigger atoms of the lower periods.

Key Features to Remember

  • Patterns in molecule structure and reactivity can be explained using Bent's rule.

  • The substituent electronegativities can be checked to see if Bent's rule is true after identifying how the hybridisation of the fundamental atom should impact a specific attribute.

  • Bent's rule states that the orbitals aimed at electropositive substituents are where the atomic s character is concentrated.

  • It is also possible to explain the non-bonding orbital hybridization using an extension of Bent's rule.

  • Contrary to VSEPR theory, whose conceptual underpinnings are currently under doubt, Bent's rule is nevertheless regarded as a key idea in contemporary bonding methods.

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FAQs on Bent's Rule

1. Explain the s-orbitals electronegativity order?

Since the s-orbital is known to be nearer to the nuclei, a hybridised orbital electronegativity rises as the s-character increases.

sp3d2 < sp3d < sp3 < sp2 < sp

2. Explain the bond angle of water?

The bond angle of water is 104.5°. As per Bent's rule, the lone pairs function as extremely electropositive substituents and possess additional s character. The bonding electrons' p character has therefore been enhanced. The higher p character causes the bond angle connecting water to become lower than the tetrahedral 109.5°.

3. Explain Bent's rule of non-bonding orbitals?

Bent's rule suggests lone pair orbitals must adopt a higher s character to stabilise the unshared, tightly bound nonbonding electrons.